MoOx-based high-density nanoarrays on a substrate via smart anodizing as novel 3D electrodes for nano-energy applications

Abstract

Nanostructured molybdenum oxide (MoO_x) has many exciting properties that are highly dependent on the synthesis procedure. MoO_x nanostructures should be aligned on a substrate for nanoelectronic on-chip applications, which has been challenging. Here, for the first time, arrays of MoO_x-based nanoprotrusions of various morphologies (nanogoblets and nanorods), dimensions (20–500 nm), and surface densities (up to 10¹¹ cm⁻²), spatially separated and vertically aligned on a Si wafer, were synthesized via self-organized porous-anodic-alumina (PAA)-assisted anodization of a Mo underlayer covered with a few nm thick Nb interlayer. This creative anodization approach enabled sustainable growth of fully amorphous MoO_x within and under the PAA nanopores in several aqueous electrolytes, which other reported methods cannot accomplish. The nanoarrays grow via the outward migration of Moⁿ⁺ cations enabled by the thin niobium-oxide interlayer, followed by the concurrent migration of Moⁿ⁺ (n = 4–6) and Nb⁵⁺ cations in the PAA barrier layer and along the pore walls, competing with the migration of Moⁿ⁺ through the anodic molybdenum oxide that grows within the ‘empty’ pores. The nanorods derived after selective PAA dissolution feature a core/shell heterostructure: The shells are composed of MoO₃, several molybdenum suboxides (Mo⁵⁺, Mo⁴⁺), stoichiometric Nb₂O₅, and Al₂O₃, all mixed at the molecular level, whereas the cores are slightly hydrated and reduced MoO₃, as revealed by X-ray photoelectron spectroscopy. The annealing in air or vacuum at 550 °C increases the oxidation state of Moⁿ⁺ cations in the shells and causes the formation of monoclinic MoO₂ and orthorhombic Nb₂O₅ nanocrystallites in the bottom oxide. Mott–Schottky analysis disclosed n-type semiconductor properties of the cores, with the charge carrier density reaching 1 × 10²² cm⁻³, whereas the shells seem more dielectric. The cyclic voltammetry and galvanostatic charge–discharge measurements featured characteristic reversible redox reactions, intensive electron transport, intercalation pseudocapacitive behavior, competitive charge-storage performance, and good rate capability of the rod's cores, which means the potential for nano-energy applications.